Lilienthal recognised the position, of course, and saw there might be peril in the use of a biplane; but he was content, none the less, to rely upon his skill. In each glide he made he became more expert; instead of allowing his machine to slip to the ground when struck by a gust, he restored its equilibrium by an instantaneous movement of his body: he was, in fact, like a man who had learned to ride a bicycle--balancing himself without pausing to think what he should do. But Lilienthal, in his navigation of the air, was facing a danger the cyclist has not to fear. He was braving dangerous wind-gusts; and he did not know, and had no means of knowing, with just what strength these gusts would strike his craft. Also--and this too was a danger no man on earth need fear--he had empty air below him should he fall. The peril grew greater as his skill increased, because he soared higher, and left greater distances between him and the ground below. In another way, also, he courted greater risk; and this was through gliding in stronger winds. At first, when he was unskilled, he had cared only to brave a wind of a velocity, say, of 10 or 15 miles an hour. But soon, feeling his balancing power grow greater, he ventured into the air when there was a wind of from 20 to 25 miles an hour. In regard to this question of the strength of the wind, uncertainty often exists. What, for example, is a “stiff breeze”? What is a “strong wind”? And at what velocity must the wind blow before it is called a gale? Such questions are often asked, and the table below should prove instructive: Velocity of wind Description given (miles per hour). such strength. 0-5 Calm. 6-10 Light air. 11-15 Light breeze. 16-20 Gentle breeze. 21-25 Moderate breeze. 26-30 Fresh breeze. 31-36 Strong breeze. 37-44 Moderate gale. 45-53 Fresh gale. 54-60 Strong gale. 61-69 Whole gale. 70-80 Storm. 80 upwards Hurricane. [Illustration: FIG. 27.--Lilienthal combating a wind-gust.]